New Oxybromide Delivers Ultrafast Oxide-Ion Conduction Inside the Norby Gap

A team of researchers in Japan and the United Kingdom has identified a crystalline material that moves oxide ions with remarkable speed at temperatures far below those demanded by today’s solid oxide fuel cells. The material, a Sillén oxybromide with the composition Bi_1.9Te_0.1LaO_4.05Br, achieves a bulk oxide-ion conductivity of 0.01 S cm⁻¹ at just 233 °C, a temperature range in which very few electrolytes have ever performed this well. The discovery, led by Professor Masatomo Yashima of the Department of Chemistry, School of Science at Institute of Science Tokyo, was published in Nature Communications on October 14, 2026, and points toward a new generation of low-temperature electrochemical devices.

Solid oxide fuel cells convert chemical energy directly into electricity and can run on hydrogen, natural gas, or biogas, producing only water as the main product when hydrogen is the fuel. Despite these advantages, conventional devices face a persistent limitation: their electrolytes must be heated to very high temperatures to conduct oxide ions efficiently. Commercially available electrolytes such as yttria-stabilized zirconia, commonly known as YSZ, typically require operating temperatures of roughly 700 to 1,000 °C. At those temperatures, materials degrade faster, manufacturing becomes more expensive, and compatibility problems arise between the different components of the cell.

Lowering the operating temperature to between 200 and 500 °C has long been a goal of materials scientists, yet the search has been unexpectedly difficult. Electrolyte materials that simultaneously combine sufficient oxide-ion conductivity, chemical stability, and negligible chemical expansion are extremely rare in this window. The temperature range has become known as the Norby gap, and bridging it is considered an important challenge not only for solid oxide fuel cells but also for a wide variety of chemical and energy-conversion processes. The new oxybromide sits squarely inside this gap, which is precisely why the researchers consider it a breakthrough.

The path to the discovery began with an earlier compound. In previous work, the same team reported high oxide-ion conduction in a related Sillén oxychloride, Bi_1.9Te_0.1LaO_4.05Cl, but its conductivity fell short of the levels needed for the Norby gap. Building on that result, the researchers replaced the chloride ion with a larger and more polarizable bromide ion, synthesizing Bi_1.9Te_0.1LaO_4.05Br and systematically investigating its crystal structure and transport properties. The substitution proved decisive, unlocking conductivity far beyond that of the oxychloride precursor.

The performance figures are striking when compared with established electrolytes. YSZ and lanthanum strontium gallium magnesium oxide, known as LSGM, require temperatures of 644 °C and 534 °C respectively to reach the same 0.01 S cm⁻¹ bulk conductivity that the new oxybromide delivers at 233 °C. The material’s oxide-ion conductivity even exceeds the proton conductivity reported for several representative proton-conducting ceramics. Beyond raw conductivity, the compound exhibits high chemical stability across a range of atmospheres, including dry and wet oxidizing, reducing, and CO₂ environments at temperatures up to 300 °C, and it shows negligible chemical expansion, a property that matters greatly for the long-term mechanical integrity of fuel cell stacks.

Crystal structure analysis revealed a Sillén-type architecture in which highly conductive triple fluorite-like slabs are sandwiched between bromine layers. This layered arrangement creates a distinctive structural environment that supports fast oxide-ion transport, and the researchers regard the sandwich structure itself as a new material-design principle. By engineering similar slabs between different halide layers, other members of the family can potentially be tuned in the same way. Consistent with this idea, the team synthesized additional Sillén oxybromides and iodides that also demonstrated high electrical conductivity, suggesting the broader family offers a promising platform for fast oxide-ion conductors.

The most surprising result concerns the microscopic mechanism of ion movement. Bi_1.9Te_0.1LaO_4.05Br contains excess oxygen, reflected in its oxygen content of 4.05, and conventional wisdom holds that such excess-oxygen conductors should transport oxide ions predominantly through an interstitialcy diffusion mechanism, in which interstitial ions displace lattice ions along the conduction path. Instead, through a combination of diffraction studies, first-principles calculations, and ab initio molecular dynamics simulations, the team found that both oxygen vacancies and interstitial oxide ions coexist in the material, with vacancies at lattice oxygen sites and excess ions at interstitial sites, and that the transport proceeds by a vacancy-mediated mechanism. Oxide ions migrate from a lattice site to a neighboring vacant lattice site, moving through vacancies despite the presence of surplus oxygen atoms and interstitial ions.

This finding challenges the widely accepted picture of how oxide ions should behave in excess-oxygen materials. “Our findings demonstrate that strategically engineered oxygen vacancies enable ultrafast oxide-ion transport even in materials containing excess oxygen atoms,” Yashima said. “This overturns a widely accepted picture of how oxide ions should move in excess-oxygen materials and opens a new direction for designing fast oxide-ion conductors.” The result suggests that materials previously expected to rely on interstitialcy diffusion may instead be optimized by deliberately introducing and controlling vacancies, expanding the design space available to solid-state ionics researchers.

The research was a collaborative effort involving graduate students Yusuke Kanenori and Hidetoshi Udagawa along with Assistant Professors Kei Saito, Yuichi Sakuda, and Kotaro Fujii from Science Tokyo; Professor Tatsumi Ishihara and Dr. Maksymilian Kluczny from Kyushu University in Japan; Professor Stephen Skinner and Dr. Mudasir A. Yatoo from Imperial College London; and Professor Kazuhiro Mori from the High Energy Accelerator Research Organization (KEK) and J-PARC in Japan. The authors declare no competing interests, and the study was an experimental investigation combining synthesis, diffraction, computational modeling, and electrochemical characterization.

Important work remains before the material can reach practical devices. The researchers note that the total conductivity of the compound is currently lower than its bulk conductivity because of grain-boundary resistance, meaning that optimization of the microstructure, including grain growth and densification, will be essential for translating the exceptional bulk performance into real electrolyte performance. Nevertheless, the team believes the discovery could lay the foundation for a new research field it calls Oxide Ionics. “This discovery establishes a new foundation for designing novel oxide-ion conductors based on vacancy-mediated transport in materials with excess oxygen atoms,” Yashima said. “By sandwiching highly oxide-ion conductive triple fluorite-like slabs by Br layers, we may be able to develop a broader family of novel oxide-ion conductors for low-temperature SOFCs, solid oxide electrolysis cells, and other technologies.” If that vision is realized, fuel cells capable of efficient operation at a few hundred degrees Celsius could move significantly closer to commercial reality.

Subject of Research: Vacancy-mediated ultrafast oxide-ion conduction in excess-oxygen Sillén oxybromides for low-temperature solid oxide fuel cells

Article Title: Novel ultrafast solid oxide-ion conductors for low-temperature fuel cells

Article References: Novel ultrafast solid oxide-ion conductors for low-temperature fuel cells. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: oxide-ion conductors, Sillén oxyhalides, solid oxide fuel cells, Norby gap, bismuth oxybromide, oxygen vacancies, ionic conductivity, electrolytes, low-temperature fuel cells, crystal structure, ab initio molecular dynamics, chemical stability